HTT and Huntington disease: For instance, familial AD can be caused by mutations in APP, PSEN1 or PSEN2, while familial ALS can be associated with mutations in SOD1, C9ORF72, TARDBP, or FUS. Another example of familial NDD is HD, caused by an expansion of the trinucleotide CAG in the gene encoding huntingtin, leading to an expanded polyglutamine stretch in the huntingtin protein.